Poor reproduction in laboratory-born squirrel monkeys has been partially attributed to lack of learning of maternal behavior through housing of single-aged groups. At the Primate Research Laboratory, monkeys are housed in multi-generational groups throughout their lives; however, reproductive performance in primigravida monkeys remains poor, particularly in 2 and 3 year old monkeys. Pelvimetry, weight and body length were determined just prior to the breeding season for female Bolivian squirrel monkeys in 4 age groups 2, 3, 4 or 5 years old) to determine if there were physical differences that could contribute to poor reproduction. Monkeys were anesthetized for pelvic radiography weight, and morphometric body measure-ments. Two year old females had significantly smaller pelvic measures than older monkeys. With this year's data it is evident that 3 year old monkeys also have significantly smaller pelvic measures than 4 or 5 year old monkeys. Two year old monkeys had slightly smaller measures of the axial skeleton. Two and 3 year old monkeys differed significantly in body weight and body mass from 4 and 5 year old monkeys. There were no significant differences in pelvimetry in pregnant and non-pregnant monkeys in the same age group. These data indicate that 2 and 3 year old monkeys are physically immature (low body weight) and have a smaller interior pelvic diameter than older monkeys. Both of these factors could contribute to poor reproduction. In this colony, low dam body weight has been associated with lower birth weight infants and increased fetal mortality.